Computerised Scheimpflug Densitometry as a Measure of Corneal Opacification following Excimer Laser Surgery

Abstract

Some degree of physiological subepithelial opacification can be photographed in all human corneas. In most patients undergoing refractive treatment with an excimer laser further opacification occurs. The Nidek EAS 1000 Anterior Segment Analysis System was used with a 6-mm Scheimpflug slit image in the vertical meridian to photograph both these types of opacities. An image-processing technique was developed to measure corneal opacification quantitatively. A 2.5-mm diameter circle centred on the vertex of the posterior aspect of the corneal image was used as the standard area for analysis. The highest grey scale intensity at which the software demonstrated at least 2% of the analysis area to be opaque is called the corneal opacity index. The average corneal opacity index increased from 70 before treatment to 110 one month after treatment. A comparison with subjective assessment found the corneal opacity index to be the superior method for assessing corneal opacification.